brain sciences Review The Significance of Tau Aggregates in the Human Brain Rudy J. Castellani 1,2 1 Department of Pathology, Anatomy, and Laboratory Medicine, West Virginia University, Morgantown, WV 26506, USA; [email protected] 2 Department of Neuroscience, Rockefeller Neuroscience Institute, West Virginia University, Morgantown, WV 26506, USA Received: 24 October 2020; Accepted: 7 December 2020; Published: 11 December 2020 Abstract: Neurofibrillary degeneration has attracted the attention of neuroscientists as both a hallmark of the disease and a subject for experimentation for more than a century. Recent studies implicate phosphorylated tau (p-tau) directly in neurodegenerative disease pathogenesis, although the human data continue to raise questions. P-tau accumulates with age in a roughly hierarchical manner, but avoids abundance in the neocortex unless co-occurring with amyloid-β. Neurodegenerative tauopathies tend to have p-tau morphologies that differ from aging and Alzheimer’s disease. Tau isoforms (3R vs. 4R) have a tendency to vary with tauopathy phenotype for unknown reasons. Selective vulnerability to p-tau and spatial-temporal disconnect from amyloid-β are evident in aging. P-tau assessment at autopsy involves tissue decomposition, which may skew microanatomical observations toward limited biological meaning. Two major consensus guidelines for interpreting p-tau at autopsy emphasize the challenges of clinicopathologic correlation, and reinforce the observation that regional neurodegeneration is a better correlate of clinical signs than is proteinopathy. Despite the proliferation of interesting and novel theories related to tau-mediated pathogenesis, the weight of the human observations suggests that neurofibrillary degeneration is an epiphenomenal hallmark of aging and disease rather than an epicenter of neurotoxicity. This is consistent with numerous tau-targeted therapeutic strategies that have been unsuccessful to date. Keywords: tau; tauopathy; neurofibrillary; Alzheimer’s disease 1. Introduction Neurofibrillary degeneration was first identified by Alzheimer in his seminal case report linking microscopic lesions to progressive neurological decline [1]. In doing so, he established Alzheimer’s disease (AD) as an entity and neurodegeneration as a category of disease. Much has been learned about AD since then, although the lesions in question remain the same. Indeed, the silver technique used by Alzheimer is still used today to identify the same hallmark lesions [2,3]. Researchers in the early and mid-20th century described neurofibrillary degeneration in copious detail, but were circumspect about its significance with respect to the disease process. Bielschowsky commented that alterations in nerve cells indicate only that “pathological processes have taken place” [4]. Malamud in 1929 concluded from his case series that “etiologically, this clinicopathologic syndrome may be caused by a variety of factors,” and that “the whole process has been so thoroughly completed that a possible pathogenetic theory of it could only be guessed at” [5]. McMenemey in 1940 suggested the changes “as a permanent tombstone to mark the site of the deceased cell” [6]. King noted in 1942 that “the nature and origin of this pathological material remain unsettled” [7]. Early reports on electron microscopy of neurofibrillary change by Kidd [8] and Terry [9,10] were purely descriptive in nature, with Kidd commenting that “it is difficult to speculate on the nature of these structures.” Brain Sci. 2020, 10, 972; doi:10.3390/brainsci10120972 www.mdpi.com/journal/brainsci Brain Sci. 2020, 10, 972 2 of 17 Hirano did not attach special significance to neurofibrillary tangles but instead listed populations of neurons that appeared vulnerable [11]. The paradigm shifted in the mid-1980s as researchers identified individual proteins within microscopic lesions. Brion et al. [12] first reported tau protein (named for its role in polymerization of tubulin [13]), as a major protein component of neurofibrillary tangles. Grundke-Iqbal et al. [14] reported similar findings shortly thereafter and commented that post-translational modifications such as phosphorylation may be driving the histogenesis of neurofibrillary change. Kinase-phosphatase biology would soon follow [15]. Both phosphatase modifiers and kinase inhibitors are in clinical trials today for Alzheimer’s disease (AD) therapeutics [16]. In essence, the hallmark lesion itself has been conceptualized in recent years as an epicenter of pathogenesis and a potential target for therapy. The purpose of this review is to explore the subject of p-tau lesions as they present in the human brain, in both aging and neurodegenerative diseases. The spectrum of p-tau lesions, which has expanded considerably in recent years, has added a level of complexity, not only morphologically, but in terms of a putative role in various disease states. The central question at issue is the nature and significance of p-tau aggregates identified at autopsy, and whether the lesions themselves and their constituent proteins drive neurodegenerative diseases. 2. Phosphorylated Tau Assessment In Situ The term “tauopathy” was first used for a condition with a mutation in the tau (MAPT) gene [17], which seems appropriate since tau protein itself is mutated in this condition and associated with autosomal dominant disease. Over time, the term “tauopathy” generalized to refer to the simple presence of p-tau aggregates identified by immunohistochemistry, whether or not the aggregates occurred in the context of clinical disease. Because p-tau occurs with age starting as early as childhood, it is important to distinguish incidental or subclinical tauopathy, i.e., tauopathy with no clear clinical correlate, from neurodegenerative tauopathy, i.e., a well-defined clinicopathologic entity with progressive neurodegeneration. (Table1). Immunohistochemical phenomena such as argyrophilic grain disease [18], aging-related tau astrogliopathy [19], primary age-related tauopathy [20], and chronic traumatic encephalopathy (2005 forward [21,22]), are examples of subclinical tauopathy insofar as they lack a consistent clinical phenotype and are diagnosed exclusively post-mortem. Progressive supranuclear palsy, corticobasal degeneration, and Pick disease are neurodegenerative tauopathies, as they invariably correlate with clinically progressive neurodegenerative disease [23]. These are sometimes referred to as “primary tauopathies” (as opposed to a “secondary tauopathy” such as Alzheimer’s disease), although it remains unclear as to whether p-tau is driving the disease process in these conditions, occurs in parallel with neurodegeneration, or is otherwise secondary to more complex biology. Exceedingly rare tauopathies with complex nosology and variable clinical presentations also appear to be neurodegenerative in nature (e.g., globular glial tauopathy, atypical Parkinsonism of Guadeloupe, diffuse neurofilament tangles with calcification [24]). Table 1. Examples of neurodegenerative tauopathy and subclinical tauopathy. Neurodegenerative Tauopathies Subclinical Tauopathies Progressive supranuclear palsy Primary age-related tauopathy Corticobasal degeneration Aging-related tau astrogliopathy Pick disease Argyrophilic grain disease Chromosome-17-linked dementia Chronic traumatic encephalopathy Guamanian dementia Neurodegenerative tauopathies represent neurodegenerative diseases in the true sense, as invariably progressive conditions with neurological signs and abundant neuropathology that includes loss of neurons. Subclinical tauopathies lack clinical features of neurodegenerative disease and are strictly autopsy diagnoses. Brain Sci. 2020, 10, 972 3 of 17 P-tau aggregates are not only age-related, but an inevitable consequence of the aging process [25,26]. They are sparse in children and young adults, but nevertheless detectable. P-tau can be strikingly abundant in middle-aged and older people in the absence of a neurocognitive disorder during life [27]. Thus, in a random autopsy sample across the spectrum of ages, p-tau aggregates are unassociated with clinical disease in the overwhelming majority of cases, with most of the rest being AD-related. Numerous different p-tau antibodies are available for brain tissue examination in situ, which reflects the large number of tau phosphorylation sites and candidate p-tau epitopes [28,29]. This is a potentially important issue because the antigenicity of different epitopes varies as a function of time and cell viability. Some antibodies better label “pretangles” (more diffuse cytoplasmic labeling with no apparent fibrils), while others may be more effective at labeling extracellular tangles (neurofibrillary tangles that persist in tissues following cell death) [30]. In practice, monoclonal antibody AT8 is commonly used because it recognizes residues in both intracellular and extracellular lesions, and does not cross react with unphosphorylated tau [30], providing robust sensitivity and a clean background. P-tau aggregates present a broad array of descriptive morphologies or intracellular “inclusions” that have expanded well beyond neurofibrillary degeneration (Table2) (reviewed in [ 31,32]). Prior to immunohistochemistry, silver reduction reactions were generally used (Figure1). The latter requires an underlying protein complex with filamentous or fibrillar structure, and were adapted empirically for the structures of interest. For example, the Bielshowsky technique which employs silver nitrate is more sensitive for neurofibrillary